>> reporter: you might want to think again. >> out on the street tonight, dr. richard besser is here. so, rich, you swear, you say it cannot give you the flu. but afterwards, if you feel that soreness, a little ache, sometimes a tiny bit of fever, it is -- >> reporter: you know, you can feel those. and it's your body reacting. it's building those antibodies. so, what i tell my patients, if you feel that sore arm and that fever, it's going to work even better for you. that can give you some comfort. >> the more you're feeling, the more you're building up antibodies. >> reporter: your body is reacting to that vaccine. >> okay, night after night, you've been saying to me, it's not too late to get the flu shot. is tonight the night it's too late, or -- >> reporter: you look all around america and people are still seeing the flu. as long as you are seeing the flu, it's still out there, there's time to be protected. maybe six weeks, maybe longer. and we are hearing the protection is about 62%. but think about it as a seat belt. it doesn't protect you against everything, but it protects you against a lot. it's a really smart way to go. >> and you're saying it begins to work just a couple of days after you get it.